1. Field of the Invention
The present invention relates to compilers for programming languages. More specifically, the present invention relates to a method and an apparatus for performing conditional compilation.
2. Related Art
Programs often include sections of source code which, in some situations, may not be desirable to compile or to execute. For example, a program might include sections of source code which make use of language or system features only available on some releases of a product. Additionally, a program might include source code that implements capabilities which should only be available to some users. Or, a program might implement debugging, tracing, or testing components which should not appear in the delivered executable code.
During the code development process, programmers often manually build and keep track of multiple source code versions, with each version containing (or not containing) the code fragments chosen for a particular version of the application. This can be problematic because the number of versions, and subsequent choices for each version, can quickly grow out of hand.
An alternative technique called “conditional compilation” is commonly used to selectively compile sections of source code. During conditional compilation, the source code to be compiled is selected from the source code file by evaluation conditions based on values supplied by the programmer or obtained from the environment. Typically, in conditional compilation systems, the programmer embeds preprocessor directives in the source code, which direct the preprocessor to select the appropriate sections of source code to compile. The modified source code from the preprocessor is then compiled by a standard compiler. (The C preprocessor cpp is an example of a preprocessor used in conditional compilation.)
However, conditional compilation is not without problems. The preprocessor may not interact properly with the language, the compiler, and other language tools (notably, debuggers). In addition, the programmer does not actually control the source code seen by the compiler; and often the programmer never even sees that final source code.
Due to the difficulties of maintaining separate code bases, and the problems with existing conditional compilation systems, many organizations write applications that avoid newer language features to ensure compatibility with older systems. The problems caused by changes in language features are so severe at some organizations that application developers at these organizations commonly refuse to consider the use of a new language feature until all fielded releases of the language support the feature. In some cases, this has caused a delay of up to five years from the time a feature is added to the language until the time it begins to be incorporated into applications written in the language.
Hence, what is needed is a method for performing conditional compilation without the problems listed above.